Modern vehicular and automotive systems are increasingly using sensors to determine whether there is adequate air pressure in tires. Direct tire pressure monitoring using in-tire pressure sensors with a radio frequency (RF) data link are used to measure tire pressure. Presently used systems need a battery power source with a life requirement of many years. Power from the battery is consumed during pressure measurement, during RF transmission and from the overall standby current of the sensor module. In order to reduce the power demand, two general operational modes have been defined depending upon whether the vehicle is in motion or is parked. As a result, a mechanism to detect wheel movement is provided. The mechanism to detect wheel movement is typically implemented either with an electro-mechanical device that is sensitive to the angular acceleration of the wheel or with a micro electromechanical sensing (MEMS) accelerometer. For example, a motion switch is implemented within the tire and relies on the angular acceleration of the wheel pressing a mechanical mass against a contact. Problems to date with such devices have been associated with the fact that with repeated impacts and vibration the electrical connection loses reliability. Some are also subject to factures of the housing and the sensor adds to the cost of the tire. Additionally, an electro-mechanical switch requires power to operate as the switch will draw current when it is conductive.
Some motion switches may have a mass that is ferrous and therefore can be activated to a closed position using a strong magnet placed against the wheel. This allows the pressure sensor to transmit more frequently as a diagnostic. However, the level of diagnostic capability using a magnet is limited and any additional diagnostic functionality, such as a low sensitivity low frequency (LF) detector, adds to the system's battery load.
An indirect method of motion detection transmits a signal to a wheel module from the vehicle chassis telling the wheel module when the vehicle is in motion. This method is indirect because the wheel module within the tire itself does not measure vehicle speed in any way. The signal can be either an RF or an LF signal but the RF implementation typically uses more power and is more costly. In the LF indirect system, an LF oscillator, a driver and a transmit coil are located inside a wheel well. On the wheel module there is a sensitive amplifier to detect the signal picked up in a small receive coil. Alignment of transmit and receive coils is sensitive in order to get the maximum angle of wheel rotation where the signal is maximized. Also, the cost associated with a LF detection indirect system is high because of the additional circuitry on the chassis side for each wheel location, but information on each individual wheel may be obtained.